(1)Field of the Invention
The present invention relates to a process for producing high-purity organic phosphines. Particularly, the present invention relates to a process for producing high-purity organic phosphines usable as a raw material in the formation of compound semiconductor thin film by the Metal Organic Chemical Vapor Deposition Method (MOCVD).
(2)Description of the Prior Art
Compound semiconductor films formed with compounds of elements in Groups III and V of the periodic table, such as indium and phosphorus, are useful as materials in the manufacture of electronic devices. There are available for forming the compound semiconductor thin film such methods as (1) Molecular beam epitaxy (MBE), (2) halide CVD, (3) metal organic Chemical Vapor Deposition (MOCVD), and (4) metal organic molecular beam epitaxy (MOMBE), etc. Of these said methods, MOCVD and MOMBE have come into wide use and attracted attention of concerned circles, since said two methods do not require high vacuum within the crystal growth system, thus readily permitting exchange of raw materials.
However, in case either one of the aforementioned methods is employed to produce compound semiconductors consisting of elements in Group III of the Periodic Table and phosphorus, which use phosphine, a highly toxic phosphorus hydride compound, as the raw material. Needless to say, the use of phosphine in substantial quantities in mass production of semiconductors is inevitably accompanied by grave concerns about environmental as well as workshop safety.
Recently, proposals have been made, with the said potential hazard in view, to use organic phosphines, such as alkyl phosphine and aryl phosphine in place of phosphine. Among those organic phosphines, in particular, monoalkyl or monoaryl phosphine are being looked upon as promising low-toxicity substitute materials for phosphine, since monoalkyl phosphine and monoaryl phosphine cause only little carbon contamination into the semiconductor thin film (e.g. J. Crystal Growth, 77 (1986) 11-18).
Among those well-known processes for producing the aforementioned organic phosphines, the following may be cited as representative examples.
(1) Reduction of an organic halogenophosphorus compound with lithium aluminum hydride or sodium. EQU 2RPCl.sub.2 +LiAlH.sub.4 .fwdarw.RPH.sub.2 +LiAlCl.sub.4 EQU RPCl.sub.2 +4Na (+2H.sub.2 O).fwdarw.RPH.sub.2 +2NaCl+2NaOH
(2) Reduction of diester of organic phosphonous acid or diester of organic phosphonic acid by lithium aluminum hydride EQU R.sup.1 P(OR.sup.2).sub.2 +LiAlH.sub.4 .fwdarw.R.sup.1 PH.sub.2
(3) Reaction of phosphine with olefin EQU PH.sub.3 +C.sub.n H.sub.2n .fwdarw.(C.sub.n H.sub.2n+1)PH.sub.2 +(C.sub.n H.sub.2n+1).sub.2 PH+(C.sub.n H.sub.2n+1).sub.3 P
(see "Shin Jikken Kagaku Koza" (A Series of Courses on New Experimental Chemistry) Vol.12, p. 421, published by Maruzen Co., Ltd., Tokyo, in 1976).
Out of these processes for synthesizing organic phosphines, in the cases of the reduction processes such as (1) and (2) as set forth above, it is known to be advantageous from the viewpoint of improving the yield to use as the reducing agent principally a compound having active hydride in its molecule, such as lithium aluminum hydride and sodium borohydride (NaBH.sub.4). However, the inventors of the present invention have discovered that the use of sodium bis(2-methoxyethoxy) aluminum hydride (NaAlH.sub.2 (OC.sub.2 H.sub.4 OCH.sub.3).sub.2) is effective for preventing contamination of organic phosphine with organic halide and/or other impurities, and hence proposed a process for producing high-purity alkyl phosphines (Japanese Patent Application Hei-2-265588).
Although sodium bis (2-methoxyethoxy) aluminum hydride is an effective reducing agent as proposed, the use thereof has posed a problem in that 2-methoxy ethanol, derived from the reducing agent, contaminates the organic phosphine.
What is more, in such reduction reaction using either one of the aforementioned reducing agents, complex intermediate compounds are formed. An acid such as hydrochloric acid is used to hydrolyze these complex intermediate compounds.
In a step of said hydrolysis process, a part of organic phosphines produced by the reduction are decomposed, whereby organic halides, such as alkyl halides and aryl halides, are produced.
Furthermore, an alkyl halide or an aryl halide which is used as the alkyl source or aryl source in the synthesis of an organic halogenophosphorus compound or an oxoacid ester of phosphorus is often entrained in the organic halogenophosphorus compound or an oxoacid ester of phosphorus and remains in the alkyl phosphine or aryl phosphine. Generally, separation purification by distillation, etc. has been hardly practicable, since the boiling points of alkyl phosphine or aryl phosphine and alkyl halide or aryl halide are close to each other.
In as much as the semiconductor manufacture requires particularly high-purity organic phosphines, it has been a requisite condition that trace contents of impurities, particularly, silicon, organic halides and 2-methoxy ethanol, be removed from the reduction product.